The present invention relates generally to the field of switch mode power supply (SMPS). More particularly, embodiments of the present invention relate to circuits for discharging input capacitors in SMPS.
Switched-mode AC-DC converters are widely used to provide stable power to home appliances, computers, communicating devices and so on. Switch mode power supplies have many advantages, including smaller volume, better stability, and higher power efficiency. As SMPS finds wide spread applications in portable consumer electronics, more emphasis has been placed on performance issues such as electromagnetic interference (EMI) protection and standby power. For EMI protection, the input port of the converters often has a capacitor connected directly across the two AC lines (X-capacitor) for suppressing noise generated. In conventional power supplies, a discharging resistor is often disposed parallel to the X-capacitors for discharging the capacitor and avoiding the user getting electric shock when the device is unplugged from the power outlet.
FIG. 1 is a simplified schematic diagram illustrating a conventional power converter. As shown in FIG. 1, converter 100 includes an input filter 101, a bridge diode BD, a capacitor C150, and a DC-DC converter. Input filter 101 includes a first X-capacitor C100, a second X-capacitor C130, a discharging resistor R110, and an inductor L120. Input filter 101 is configured to stabilize the AC power source Vac, and to reduce electromagnetic interference (EMI). The bridge diode BD includes two input terminals and two output terminals and four diodes. The two input terminals of bridge diode BD are connected to two output terminals of the inductor L1. The two output terminals of the bridge diode BD are connected to two terminals of capacitor 150. Bridge diode BD rectifies the AC power source, and capacitor 150 is used to smooth the output from the rectifier to generate a DC voltage Vbulk to the DC-DC converter. The DC-DC converter is configured to convert voltage Vbulk into a regulated DC voltage Vo.
In input filter 101, two terminals of the first X-capacitor C100 and two terminals of discharge resistor R110 are connected to two terminals of the AC power source Vac, respectively. Two input terminals of inductor L120 (also referred to as a choke coil) are connected to two terminals of the AC power source Vac, and two output terminals of the inductor L120 are connected to two terminals of the second X-capacitor C2. When the power supply is in operation, capacitors C100 and C130 are charged by Vac, and input filter 101 stabilizes the AC power source Vac and reduces electromagnetic interference (EMI). When supply of the AC source Vac stops, for example, after a power supply is unplugged from the power outlet, it is necessary to discharge of the two capacitors. As shown in FIG. 1, conventional power supply 100 has the discharging resistor R110 to discharge capacitors C100 and C130. However, since the discharge resistor R110 continues to consume power when the AC power source Vac is on, it reduces efficiency of the power system and causes large standby power. Under no load condition, this standby power can constitute a substantial portion of the power consumption of the power supply.
Various methods have been proposed to reduce this power loss by introducing a detection circuit to determine whether the AC power source has stopped and only use the discharge resistor to discharge the X-capacitors when the AC power source is stopped. For example, a conventional detection circuit compares a sampled AC voltage with a predetermined constant reference voltage. An alternative conventional detection circuit uses a reference voltage that is a fixed fraction of a peak voltage of the sampled AC voltage.